In this chapter we describe an optimized eosinophil transmigration assay. Transmigration of purified human peripheral blood eosinophils can be studied using special insert with membrane coated with extracellular matrix components or membrane covered with cells growing as a confluent monolayer, such as vascular endothelial cells of any origin or airway epithelial cells. In our opinion, eosinophil transmigration assay performed through monolayer of human microvascular endothelial cells of lung origin is a suitable tool to estimate the full migratory potential of eosinophils in studies on the pathology of asthma or other respiratory diseases, where eosinophils play important effector functions. This experimental system is easy to perform, simple for optimization, and comparable to in vivo processes occurring during eosinophil migration to the inflammatory sites in lungs.